This invention relates to a method and system for medical imaging. More particularly, this invention relates to a method and system for extended volume imaging using a Magnetic Resonance Imaging (MRI) system and employing a moving patient table.
Certain clinical situations require that a head-to-toe scan of a patient be made. For example, metastatic cancer can occur anywhere in the body and patients at risk for metastatic disease need to be evaluated regularly. Currently head-to-toe or, alternatively, extended volume imaging is generally performed with whole body positron emission tomography (PET) systems or nuclear studies in which small amounts of radioactive substances are given to the patient and allowed to collect in regions of rapid tumor growth. The imaging sensitivity and specificity of magnetic resonance (MR), however, makes MR a more desirable choice for diagnosis particularly when Gadolinium (Gd) contrast agents are employed. Gd contrast agents are typically administered intravenously and tend to collect in regions of angiogenesis associated with active tumors. Unfortunately, MR scanners have limited sensitive volumes and whole body scanning requires that a series of images be made at different stations. Patient motion and table registration can make the joining of these separate images a challenge and tend to create image artifacts referred to as “stitching artifacts”.
Generally, imaging using a MRI system involves imaging a volume of interest in a MRI scanner's usable volume. The usable volume is defined as a contiguous area inside the patient bore of a Magnetic Resonance scanner and it can be limited in size. Typically, when the usable volume fails to cover an extended object, a method for examining the whole volume containing the object employs repeated executions of positioning and imaging a fraction of the whole volume within the scanner's usable volume to obtain regional images. A subsequent assembling operation then assembles or “stitches” the regional images together to produce a final image of the whole volume of interest. Such an approach is typically challenged by the “stitching” artifact issue wherein resulting final images often suffer from distinctive artifacts at the boundaries of the “stitched” pieces.
Existing techniques achieve correct combination of regional images through full spatial encoding along patient table motion direction. With other existing methods, the patient table is held stationary while data is collected and moved between the collection of the regional images. These techniques minimize “stitching” artifacts by using slab selection profiles that are as rectangular as possible, and/or discarding image data near the boundaries. As a result, these techniques tend to be inflexible, require prolonged radio frequency (RF) excitation, and involve considerable acquisition efficiency degradation.
More recently, imaging an extended volume is performed by means of simultaneous patient table translation thereby allowing examination of a field of view that extends beyond the usable volume of an MR scanner. It is however very difficult to achieve 3-dimensional whole body coverage with favorable mapping accuracy, spatial resolution, signal-to-noise ratio and total scan time.
Parallel imaging offers a way to speed up conventional MR imaging. The idea of detecting MR signal with multiple coils receiving in parallel has been explored. Recent advances represented by simultaneous acquisition of spatial harmonics (referred to as SMASH) and sensitivity encoding (SENSE) make up for a reduced number of gradient-driven spatial encodes by integrating data from an array of receive RF coils. SMASH and the likes assume a frequency perspective they fill up skipped k-space lines through approximating Fourier harmonics with linearly combined coil sensitivity profiles. SENSE and related methods adopt a space perspective they resolve localization ambiguities through algebraically extracting additional spatial information encoded with the sensitivity profiles.
What is needed is a method and system for extended volume imaging, such as head-to-toe imaging, using a MRI system to reduce the imaging time while offering good image quality.